Question

• Use the Clausius-Clapeyron equation to calculate the temperature (°C) at which pure water would boil at a pressure of 400.0 torr.

• Temperature = ___°C

• the vapor pressure of h20 at this temperature is _____ torr.

Answer 1

According to Clausius - Clapeyron equation

ln(P₁/P₂) = (∆H_vap/R)(1/T₂ - 1/T₁)

P₁ = Vapor pressure 1, 760torr (when a liguid boils at normal boiling point when its vapor pressure reaches atmospheric pressure )

T₁ = Temperature 1, 373.15K ( normal boiling point of water)

P₂ = Vapor pressure 2, 400torr

T₂ = Temperature 2, ?

∆H_vap = Enthalphy of vaporization of water, 40.66kJ/mol

R = gas constant, 0.008314kJ/mol K

substituting the values  

ln( 760torr/400torr) = ( 40.66kJ/mol/0.008314kJ/molK)(1/T_{2 ^-} 1/373.15K)

0.642 = 4890.6K( 1/T₂ - 0.002680K^-1)

0.642 = 4890.6K /T₂ - 13.11

4890.6K /T₂ = 13.752K

T₂ = 355.63K

T₂ = 82.48℃

Therefore,

Temperature at which pure water would boil at a pressure of 400 torr = 82.48℃

The vapor pressure of H2O at this temperature = 400torr